1. Field of the Invention
The present invention relates to a power supply circuit and a method of driving the same and, more particularly, to a power supply circuit which includes a charge pump circuit and generates a predetermined voltage, and a method of driving the same.
2. Description of the Related Art
Of various types of circuits, some circuits such as display driving circuits require a plurality of power supply voltages when they are driven. An example of a power supply circuit which supplies a plurality of power supply voltages is a power supply circuit including a charge pump circuit. This power supply circuit has one or a plurality of capacitors, and adds capacitor charge voltages to generate another voltage.
This charge pump type power supply circuit used in a display driving circuit and the like is so designed that the connections between the capacitor and a plurality of power supply voltages are periodically switched. This switching of the connections between the capacitor and a plurality of voltages is controlled by on/off operation of switches.
FIGS. 4A and 4B are schematic views showing an example of a conventional charge pump type power supply circuit.
As shown in FIG. 4A, a power supply circuit 900 includes switches SW1 to SW4 and capacitors C1 and C2. The switch SW1 has one terminal to which a voltage VCC is applied, and the other terminal connected to a terminal C1M. The switch SW2 has one terminal connected to the terminal C1M, and the other terminal to which a voltage VSS (GND) is applied. The switch SW3 has one terminal to which a voltage VDC is applied, and the other terminal connected to a terminal C1P. The switch SW4 has one terminal connected to the terminal C1P, and the other terminal connected to a terminal VOUT. The capacitor C1 has one terminal connected to the terminal C1M, and the other terminal connected to the terminal C1P. The capacitor C2 has one terminal connected to the terminal VOUT, and the other terminal to which the voltage VSS is applied.
In the power supply circuit 900 as shown in FIG. 4B, in a first period, the switches SW2 and SW3 are turned on, and the switches SW1 and SW4 are turned off. Since the potential of the terminal C1P is set at VDC and that of the terminal C1M is set at VSS, the capacitor C1 is charged to the voltage VDC.
Then, in a second period, the switches SW1 and SW4 are turned on, and the switches SW2 and SW3 are turned off. Accordingly, the potential of the terminal C1M is set at VCC, and the terminal C1P is connected to one terminal of the capacitor C2. Since the voltage VDC is held in the capacitor C1, the potential of the terminal C1P becomes (VDC+VCC), so a voltage (VDC+VCC=VGH) is applied to one terminal of the capacitor C2 to charge it to the voltage VGH. As a result, the output terminal VOUT outputs the voltage VGH.
Then, when the switches SW1 and SW4 are turned off and the switches SW2 and SW3 are turned on again in the first period, the voltage in the capacitor C2 is held, and the output voltage from the output terminal VOUT is also maintained. By periodically charging the capacitor C2 by repeating the above operation, the power supply circuit 900 can supply a predetermined voltage from the output terminal VOUT.
In the charge pump type power supply circuit as described above, in the first and second periods, a transient current flows from the power supply to each capacitor albeit for a short time during a period immediately after each switch is turned on to apply each individual voltage to the capacitor. This transient current is a very large electric current if the line between the capacitor and power supply has a low resistance. If this transient current is generated in the charge pump type power supply circuit, latch-up occurs in transistors forming the switches or in a control circuit, thereby making the power supply circuit inoperable. Also, the large electric current flowing from the power supply circuit causes defective operation of the power supply, so an operation error of the circuit occurs.